Lathes and milling machines are used for cutting external keyways and screw or drill threads, facing and boring operations, straight and taper milling etc., on a workpiece held in the machine.
However the only known machine able to cut internal keyways, whether straight or of progressive depth is a type of reciprocating chisel which cuts bit by bit while moving along the workpiece. Moreover this machine is very expensive and must be properly centered relative to the workpiece.